1. Switch it off fully and switch it back on again
2. Try a different charging cable, make sure its an original Apple cable.
3. Try a different computer/wall socket plug to narrow down the fault
4. If all of those fail to get any connection, go to an Apple store/call Apple support.

If you go to the store, you should book a Genius appointment in advance in case they're busy and can't see you without an appointment.